1. Field of the Invention
This invention relates generally to apparatus for precision molding of optical attenuators and related control apparatus operating as a system and the methods employed by such systems. In particular, this invention relates to a high-precision injection mold having certain adjustment features to control at least one dimension of an injection molded article, and to a control system and methods for controlling the adjustment of the mold so that the molded articles meet defined dimensional criteria.
2. Description of the Prior Art
U.S. Pat. No. 5,082,345 describes a fiber optic connector that includes an optical attenuator element for adding attenuation and reducing return loss in the fiber optic system. The optical attenuator element includes a disk-shaped plate-like lens portion through which light travels from one optical fiber to another length-wise adjacent optical fiber. The lens portion of the optical attenuator element is supported in position by a sleeve of the fiber optic connector that includes a longitudinally extending slot. The attenuator element is generally T-shaped with the stem of the T forming the lens portion and the capital portion of the T forming a head, which is captured in the longitudinally extending slot in the fiber optic connector. To achieve the desired result the lens or disc must be selected of a suitable thickness, flatness, surface finish, and parallelism to maintain fiber-end contact and to provide the desired attenuation. The lens portion of the attenuator generally has a diameter of about 1.25 mm and a thickness which ranges from about 200 to 1750 microns to achieve a 5 dB to a 20 dB loss, respectively. Using polymethylmethacrylate (PMMA) plastic, the attenuator elements can be molded in various thicknesses to attain desired attenuation range, but this requires a high-precision molding system to achieve the desired result. The preferred plastic is Acritherm HS312s PMMA available form ICI Acrylics, Inc.
In practice, the manufacture of such molded optical attenuators presents many difficulties. It is desirable that the lens portion be manufactured to provide attenuators in 1 dB increments, and so labeled, so that the attenuators can be easily and reliably used in the field without the necessity of testing. To achieve the desired reliability, the thickness of the lens portion must be held to about xc2x11.3 xcexcm. This close tolerance on thickness is difficult if not impossible using conventional molding techniques since the thermal variations that typically occur during the injection molding process are such as to cause expansion and shrinkage of the molding space by amounts that may be significantly greater than this, depending on the nature of the materials forming the mold.
Additionally, it is desirable to form attenuators down to 1 dB which amounts to a total thickness of only about 50 xcexcm, which some have characterized a xe2x80x9ccontrolled flashyxe2x80x9d. The total volume of the attenuator including the head portion is so small that as little as 1.5 mg of plastic forms the entire device, which can amount to one pellet or less of the plastic. Thus the processing practice has been to provide an additional waste part of much larger volume unitarily coupled to the attenuator head portion, which is then removed subsequent to the molding process. It is also desirable, that the formation of the attenuators be achieved in a highly automated process, requiring little human oversight or attention so that the per-unit cost can be held to a minimum.
Accordingly, a system for molding according to the present invention is intended to produce an optical attenuator that includes a lens portion and a supporting portion unitary with the lens portion. The system includes a molding machine containing a mold in which the attenuator is formed, an extractor robot and optical inspection portion that receives the attenuator following formation, a controller that can be in the form of a programmed general purpose computer that assesses the inspection information received from the optical inspection portion and send control signals to the molding machine and mold.
The mold includes a cavity element and a core element that together define a space in which said supporting portion of the attenuator is formed. A portion of the cavity element also defines one surface of the lens portion of the attenuator. The cavity element and core element are separable from each other by an actuating motor of the molding machine subsequent to each injection cycle to permit removal of each optical attenuator molded between the cavity element and the core element. The mold also includes a core pin, movably located in the core element. The core pin has a proximal end defining a second surface of the lens portion and a distal end remote from the molding space. A core pin motor is coupled to the core pin, through a number of intermediate elements, so that the core pin can be moved relative to the core element to adjust a thickness of the lens portion.
The mold preferably includes a core pin holder block contacting or holding the distal end of the core pin. A ball screw end cap is fixed to the core pin holder block and a ball screw is engaged in the ball screw holder block so that rotation of the ball screw relative to the ball screw end cap causes movement of the core pin holder block to adjust the position of the core pin relative to the core element, and thereby the size of the space in which the lens portion of the attenuator is molded. A first pulley is fixed to the ball screw and a second pulley fixed to the core pin motor. A transmission coupling such as a timing belt couples the first and second pulley so that rotation of the core pin motor is transferred to the ball screw for adjusting the position of the core pin relative to the core element. With a very fine pitch threading on the ball screw, exceptionally small movements can be made in the core pin that are effective to achieve the very close tolerance needed to mold the lens portion of the attenuator.
The extractor is situated adjacent to the molding machine so that it can extract each optical attenuator from the mold at the completion of each molding cycle. The extractor includes an optical measuring device preferably in the form of a laser. The optical measuring device measures at least the thickness of the lens portion and generates an output signal indicative of the measured thickness. Preferably, the measurement is one that tests the attenuation achieved through the lens portion at a specified wavelength.
The controller can be a general-purpose computer such as a p.c. that has an input coupled to the optical measuring device to receive the measurement information derived by that device. The controller has an output coupled to the core pin motor to provide a signal specifying any movement of the core pin motor necessary to achieve a desired thickness of the lens portion. The controller also has a second output coupled to the actuating motor of the molding machine providing a second signal for initiating an injection cycle by the injection molding machine. The controller can, of course, have other connections that may control, for example, the plastic injection temperature, the packing time, the shot size, the mold open and close speed, and coordinate the movements of the extractor with the mold opening. The controller can also use the measured output of the measuring step to control an engraver focused on the supporting portion of the attenuator to engrave a symbol indicative of the measured result of the optical measurement. This information can also be employed to control a sorting mechanism to collect the optical attenuators into prescribed groups based on the result of the optical measurement.
The controller can be supplied with a stepped index or table of optimum thickness values corresponding to selected values of attenuation desired for the lens portion. The index or table can also specify an acceptable value on each side of each step in the stepped index that defines an acceptable range of variability, with values outside the specified range requiring corrective action. The controller preferably includes a comparitor that compares the result of said optical measuring step with a selected one of the stepped index values and associated ranges. The controller then generates a signal to the core pin motor based upon the amount to which the result of said optical measuring step differs from the desired range for the selected stepped index value.
Additional features and advantages will become apparent from the following description of a preferred embodiment of the present invention that references the accompanying drawings.